This invention relates to a system for controlling the attitude of a vehicle, particularly a spacecraft, wherein the system employs thrusters operative by an on/off control process to obtain pulsed mass expulsion and, more particularly, to pulsed mass expulsion with adaptive pulse width modulation to minimize a frequency of pulsation.
In the employment of spacecraft in a mission, such as the encirclement of the earth by a communications or scientific satellite, it is necessary to stabilize the attitude of the spacecraft. The stabilization is accomplished by overcoming the destabilizing torques of sources of disturbance to the stabilization of the spacecraft. Sources of destabilizing torque include aerodynamic torques experienced by spacecraft travel in a low orbital path through residual atmosphere, solar torque caused by pressure from the sun, and torque induced by gravity gradient from the earth's gravitational field.
The attitude control system may employ thrusters which, upon activation, develop forces and moments which push the spacecraft back to the desired attitude. A thruster may be constructed to expel mass, such as ionized particles accelerated by an electrostatic field, or molecules of gas expelled from a canister of pressurized gas. In addition, a control system employed for attitude stabilization may employ magnetic forces, as by use of magnetic torquers. The magnetic torquers comprise rods of magnetic material encircled by coils excited with electric current provided by photocells onboard the spacecraft, wherein the magnetic forces of the coils interact with a relatively weak magnetic field of the earth. The interaction of these magnetic forces develops a torque which tends to aid in the attitude stabilization.
However, the magnetic forces are unable, both in terms of magnitude and direction, to overcome the net destabilizing torque of one or more of the foregoing sources of destabilizing torque and, therefore, are useful only in reducing the rate of drift of the spacecraft away from the desired attitude. With respect to the direction of the magnetic forces developed by the magnetic torque rods, it is noted that such forces are available only in two directions, both of which are orthogonal to the earth's magnetic field, while the development of forces in three dimensions, such as the forces of an array of thrusters, is required to stabilize the spacecraft.
Of particular interest herein is the employment of the thruster which operates by expelling gas in the form of pulses of the gas. In such a thruster, the gas is contained compressed in a canister, and the canister of compressed gas communicates via a solenoid-operated valve to an exterior nozzle through which the expelled molecules of gas are directed into the environment outside the spacecraft. Use of the solenoid to open the valve during the time interval of the resulting jet, followed by a closing of the valve to terminate the jet of compressed gas, provides the desired impulse of the expelled gas.
Generally, in a control system, the pulses of the expelled gas have a fixed predetermined duration. The repetition frequency of the pulses is sufficiently low such that information obtained from attitude sensors onboard a spacecraft can be employed to monitor and predict the progress in correction of the spacecraft attitude. Thereby, the pulses of expelled gas can be generated as needed for correction and/or stabilization of the spacecraft attitude.
It has been found that a spacecraft stores sufficient gas for operation of the thrusters during a period of years which constitute the mission of the spacecraft. However, such a thruster experiences a problem in that the use of the valve, with its numerous openings and closings, produces a degradation of the valve seat and/or is mating element with a consequent impairment of the operation of the thruster. Even though a valve may be designed, possibly for a million operations, the duration of the spacecraft mission may have to be curtailed because the valve has exceeded its design lifetime in terms of the number of openings and closures.